The Federal Highway Administration (FHWA) defines a roundabout as a type of circular intersection with yield control of entering traffic, islands on the approaches, and appropriate roadway curvature to reduce vehicle speeds.

Modern roundaboutsare different from rotaries and other traffic circles. For example, roundabouts are typically smaller than the large, high-speed rotaries still in use in some parts of the country. In addition, roundabouts are typically larger than neighborhood traffic circles used to calm traffic. At a modern roundabout, entering traffic yields rather than stops to allow more efficient traffic flow through the intersection.